This invention generally relates to transmission line filters and more particularly to interdigital stripline filters having a tunable low side transmission zero.
The interdigital filter is a particular type of transmission line filter. Its application to different type filter constructions is well known in the art of electromagnetic filter design. The invention is disclosed in connection with a stripline interdigital filter. They are small sized filters which can be implemented at low cost.
A stripline filter is a modification of the basic resonant cavity. The stripline filter makes use of a series of flat conductive strips placed within a square or rectangular grounded cavity or between two ground planes. Electical coupling between the conductive strips is achieved by means of fringing electromagnetic fields associated with each strip. The fringing electromagnetic field of a single strip affect adjacent strips to a degree dependent upon the physical distance between two two adjacent strips. Each conductive strip defines a pole in the transfer function of the stripline filter. In such filters, the exact frequency of the pole depends upon the relative configuration of the conductive strips which compose the filter and the dielectric constant of the material occupying the space between the strips.
Stripline filters have long been known to have uses in miniature electronic devices, especially high frequency communication equipment. Quite often, stripline filters are used as front end filters in UHF communication devices. The function of a front end filter is to pass the desired signal frequency and attenuate all other frequencies, particularly the image frequency produced in the mixer of a receiver.
In the field of communications electronics there is a great concern about the effect of the image frequency on the standard superheterodyne receiver. The image frequency is an electromagnetic signal at a particular frequency that can cause interference problems in a superheterodyne receiver. The mechanism whereby image production takes place may be explained in the following manner: When two signals are combined, as they are in a receiver mixer, one of arbitrary frequency f (the received signal), and the other of constant frequency f.sub.LO (internal signal), resultant signals are produced at the sum and difference frequencies, f+f.sub.LO and f-f.sub.LO. Of concern are only those frequencies, f, which differ from f.sub.LO by a predetermined frequency called the intermediate frequency or f.sub.IF. There are two frequencies f which have this special relationship, f.sub.1 =f.sub.LO +f.sub.IF and f.sub.2 =f.sub.LO -f.sub.IF. As a result, without a front end filter a receiver's mixer will produce a resultant signal on frequency f.sub.IF which is equally strong for received signals at both f.sub.1 and f.sub.2. Hence either of these two signal frequencies may be picked as the signal on which the desired information is encoded. Once f.sub.1 or f.sub.2 is chosen, signals on the unchosen frequency (f.sub.2 or f.sub.1) constitute an interference if the response to the unchosen signal is not eliminated by the front end filter before it reaches the receiver mixer. The signal, f.sub.1 or f.sub.2, which is not encoded with the desired information is commonly called the image signal at the image frequency. The problem of elimination of the response of the superheterodyne radio to the image signal and its relation to the invention is more fully explained in connection with FIGS. 1 and 2.
Stripline filters, as used in UHF communication equipment for front end filters, are particularly important for attenuating the undesirable image frequency. The amount of attenuation at the image frequency is commonly called the image protection of the receiver and it is usually specified in decibels. The performance of the front end filter can be very important since it almost entirely determines the quality of the image protection in a receiver.
An important consideration in front end filter design is the selectivity, and the insertion loss of the filter at the resonant frequency. Increased selectivity in a stripline filter, necessarily increases the image protection. Normally, to increase the selectively of a interdigital filter the conductive strips of the stripline filter must be repositioned or additional quarter wavelength strips must be added to the existing filter. Either approach results in an increase of the characteristic insertion loss of the stripline filter. In a particular communications application, if a high degree of selectivity is demanded of the front end filter performance, then the system specification must be satisfied with a certain amount of insertion loss in the filter. As a result of this, the degree of attenuation of the image frequency is limited by the amount of insertion loss in the stripline filter which can be tolerated by the overall system. Therefore, precise control of the attenuation of the image frequency using a stripline filter was possible only by accepting less rigorous requirements in other aspects of the filter design.
Usually when both selectivity and insertion loss are subject to rigid requirements in a system design, the designer must abandon the use of stripline filters and resort to the more expensive and larger helical resonators in order to achieve the front end filter performance demanded by the system. Such a switch in front end filter design has in the past been unavoidable when a certain combination of high performance characteristics are required. This design modification is undesirable since stripline filters are much cheaper to manufacturer and much more reliable than the cumbersome construction of the helical resonators. Moreover, helical resonators are difficult to reproduce accurately, whereas stripline filters can be reproduced with great accuracy through the use of the well-known process of photolithography.
It is in this respect that the stripline filters in the prior art have been inadequent to give the design engineer flexibility in the implementation of a front end filter in a communications device. The addition of tunable transmission zeros in the transfer function of a stripline filter would considerably increase the flexibility of a stripline filter in virtually all applications by allowing an increase in attenuation at a selected frequency (preferably the image frequency) without a prohibitive increase in insertion loss.
Therefore, it is an object of this invention to provide a transmission line filter with improved attenuation at a selected frequency.
Additionally, it is also an object of this invention to provide one or more tunable transmission zeros in a interdigital filter to increase the quality of the attentuation performance of a interdigital filter comprising three or more quarter wavelength conductive strips.